The present invention relates to a steel sheet such as hot-rolled steel sheets and cold-rolled steel sheets, and to a method for manufacturing the same.
Steel sheets such as hot-rolled steel sheets and cold-rolled steel sheets are used in wide fields including automobiles, household electric appliances, and industrial machines. Since these steel sheets are subjected to some processing before use, they are requested to have various kinds of workability.
Recently, the request of manufacturers of automobiles, household electric appliances, industrial machines, and the like relating to rationalization becomes severer than ever, particularly in the request for improvement in production yield. To cope with the requirement, the materials thereof are requested to have particularly high homogeneity and high workability level.
Regarding the workability requested to the hot-rolled steel sheets and cold-rolled steel sheets, high tension materials (high tensile strength hot-rolled steel sheets) having strengths of 340 MPa or higher class and for the uses other than deep drawing, for example, are required to have high stretch flanging performance during burring. The cold-rolled steel sheets having strengths of 440 MPa or lower and for the drawing uses are requested to have high r value and high breaking elongation.
In recent years, the quality requirement of the consumers to the steel sheets has continuously been increasing, so that not only further improvement in the above-described workability but also homogeneity in mechanical properties in coiled products are strongly requested.
Responding to these requirements of consumers, several measures have been studied. For example, in view of the homogeneity of material quality, JP-A-9-241742, (the term xe2x80x9cJP-Axe2x80x9d referred herein signifies xe2x80x9cUnexamined Japanese Patent Publicationxe2x80x9d), discloses a method for improving the homogeneity of mechanical properties in a hot-rolled coil by adopting continuous hot-rolling. The method is a technology that uses a process of continuous hot-rolling to improve the material quality of the rolled steel sheet at front end thereof and at rear end thereof, and to eliminate the dispersion in material quality within a coil.
As for the improvement in workability of high tension materials, JP-B-61-15929 and JP-B-63-67524, (the term xe2x80x9cJP-B-xe2x80x9d referred herein signifies xe2x80x9cExamined Japanese Patent Publicationxe2x80x9d), disclose a method to improve the workability of high tension hot-rolled steel sheet by controlling the cooling speed after the hot-rolling and controlling the coiling temperature.
For the improvement in workability of IF steels (Interstitial-Free Steels), JP-A-5-112831 discloses a method to apply strong drafting during hot-rolling and to apply rapid cooling. The technology intends to improve the r value of cold-rolled steel sheet by applying final reduction in thickness of hot-rolling to 30% or more and by applying rapid cooling immediately after completed the rolling, thus reducing the grain size in the hot-rolled steel sheet.
All the above-described technologies, however, could not obtain steel sheet that is superior in both the workability and the homogeneity in mechanical properties. For example, the material properties (determined at center portion of coil width) obtained by the technology described in JP-A-9-241742 aiming at elimination of dispersion of material quality in a coil gave variations of tensile strength (TS) in an approximate range of from 4.5 to 6.3 kg/mm2 for the steel sheets of 30 to 70 K class, which range is not satisfactory for users"" requirement.
The technology described in JP-B-61-15929 aiming at the improvement in workability of high tension materials improves the balance of strength and ductility compared with conventional steel sheets, but fails to substantially solve the stretch flanging performance. Furthermore, the technology cannot improve the surface defects. Similarly, the high tension hot-rolled steel sheets manufactured by the method of JP-B-63-67524 cannot substantially solve the stretch flanging performance, though the breaking elongation and the toughness of steel sheets are improved.
Also the method described in JP-A-5-112831 aiming at the improvement in workability of IF steels cannot reduce the dispersion of material quantity to a satisfactory level. That is, according to the description of Examples of JP-A-5-112831, the average cooling speed immediately after the rolling, which average cooling speed is a feature of the invention, is in a range of from 90 to 105xc2x0 C./sec during 1 second after starting the cooling, and from 65 to 80xc2x0 C./sec during 3 seconds after starting the cooling. With that level of cooling speed, however, it was found that, under the hot-rolling condition in commercial apparatuses, the all grains in the steel sheet, particularly those in rolling top portion, cannot be refined.
The cause is presumably that the cooling cannot be started immediately after completed the finish-rolling, and there needs a time to start cooling. Since the cooling unit cannot be installed at directly adjacent to the exit of the final rolling stand owing to the necessity of installing finish thermometer and instruments to the exit of the final stand of finish-rolling mill, the cooling cannot be performed within, for example, 0.1 second after the completion of the finish-rolling. Particularly at the rolling top portion, high speed travel is not available and the rolling speed is slow, which results in long time before starting the cooling. Thus, the cooling at a cooling speed described in the patent disclosure cannot prevent the formation of coarse austenitic grains.
As described above, the top portion of the steel strip after the hot-rolling is difficult to be rapidly cooled, thus the grains cannot be fully reduced in their size, which fails to obtain superior mechanical properties and homogeneity thereof. Increased reduction in thickness in the final pass of hot-rolling is favorable for reducing the size of austenitic grains. However, increase of the reduction in thickness to 30% or more as in the technology described in JP-A-5-112831 is difficult to be actually implemented because the insufficient shape of steel sheet likely occurs.
The automobile industry has a strong need of weight reduction. Accordingly, the use rate of high strength steel sheets has been increased. To this point, the high tension materials are inferior in workability to the mild materials of 270 MPa class, thus there occur problems of production yield (cracks generated during press-working) and of quality dispersion. Consequently, the improvement in workability which is a basic characteristic of material quality is requested.
Regarding the workability, high tension materials having 340 MPa or higher tensile strength, for example, are requested for hot-rolled steel sheets and cold-rolled steel sheets to have high stretch flanging performance during burring. In addition, in recent years, the automobile application is requested to satisfy the collision safety as one of the critical characteristics, thus the steel sheets are requested to have excellent shock resistance (high shock absorption energy as an evaluation item of collision safety).
As for the improvement in workability of high tension materials, there is a prior art, Japanese Patent No. 2555436. According to the disclosure of the patent, a Ti base precipitation hardening steel is processed at cooling speeds of from 30 to 150xc2x0 C./sec after the finish-rolling, at coiling temperatures of from 250 to 540xc2x0 C., thus improving the stretch flanging performance of high tension steels of 50 to 60 K class utilizing the formed (ferrite+bainite) structure. However, the cooling speeds of from 30 to 150xc2x0 C./sec after the finish-rolling cannot be said to substantially improve the stretch flanging performance, and, there is a problem of low breaking elongation owing to the low temperature level of coiling.
JP-B-7-56053 discloses a method to improve the stretch flanging performance of hot dip zinc-coated steel sheets as the substrate of hot-rolling sheets using (ferrite+pearlite) steels of 45 to 50 K class applying cooling speeds of 10xc2x0 C./sec or more (Examples gave max. 95xc2x0 C./sec) after the hot-rolling finishing. The cooling speed is, however, 95xc2x0 C./sec at the maximum, and substantial improvement in the stretch flanging performance cannot be attained.
JP-A-4-88125 discloses a method to improve the stretch flanging performance of the high tensile materials of 50 to 70 K class using (ferrite+pearlite) steels with the addition of 0.0005 to 0.0050% Ca, applying hot-rolling at high temperatures of (Ar3 transformation point +60 to 950xc2x0 C.), and applying cooling within 3 seconds after the hot-rolling at cooling speeds of 50xc2x0 C./sec or more, preferably 150xc2x0 C./sec or less, then the cooling is stopped at temperatures of from 410 to 620xc2x0 C. depending on the composition of the steel, followed by air cooling and coiling at 350 to 500xc2x0 C. of coiling temperatures. Since, however, slight amount of addition of Ca requires an RH degassing step in the steel making stage, the steel making cost increases. Furthermore, even with the cooling condition after the hot-rolling, which cooling is a feature of the technology, the stretch flanging performance cannot be drastically improved. In addition, low coiling temperature results in low breaking elongation.
As described above, all these prior art technologies cannot attain satisfactory characteristics of stretch flanging performance and breaking elongation, and furthermore, they did not describe the improvement in the shock resistance.
As for the manufacturing of high tension steel sheets, there are methods to secure strength without adding large amount of alloying components: the method to strengthen the cooling after rolling; and the method to reduce grain size. The latter method particularly improves not only the strength but also the toughness, so that there are many proposals of the method, including JP-A-58-123823.
JP-A-61-73829 discloses a method that combines the method to strengthen the cooling after rolling with the method to reduce grain size, and the feature of the method is to apply rapid cooling to the steel sheet, which was once prepared to fine microstructure under an adjustment of rolling condition, for further reducing the grain size. That is, the rapid cooling is given to a state that slight amount of ferritic grains were generated during or immediately after the rolling, thus to finely divide the transformed structure using the ferrite to create very fine microstructure, which gives steel sheet having high strength and high toughness.
The method, however, absolutely requires the precipitation of ferrite during or immediately after the rolling owing to the low temperature rolling. Therefore, there are problems of, when the rolling finishing temperature and the temperature to stop cooling varied in the rolling width direction or in the rolling longitudinal direction, the strength varies even in the same composition steels and in a coil, which fails to attain specified strength.
As described above, since the prior art intends to refine the grains by rolling followed by rapid cooling the microscopic structure of the steel sheets to secure high strength and high toughness. Owing to the method, the prior art likely induces unstable characteristics under the variations in manufacturing conditions.
First, it is an object of the present invention to provide a method for manufacturing steel sheet that is applicable for press-working requiring strict dimensional accuracy, provides superior workability including stretch flanging performance, gives uniform mechanical properties and various levels of characteristics, and gives excellent sheet shape property.
To attain the object, the present invention provides a method for manufacturing steel sheet comprising the steps of: forming a sheet bar; forming a steel strip; applying primary cooling and secondary cooling to the steel strip; and coiling the cooled steel strip.
The step of forming the sheet bar comprises rough-rolling a continuously cast slab containing 0.8% or less C by weight.
The step of forming the steel strip comprises finish-rolling the sheet bar at finishing temperatures of not less than (Ar3 transformation point xe2x88x9220xc2x0 C.).
The step of cooling the steel strip comprises cooling the finish-rolled steel strip at cooling speeds of higher than 120xc2x0 C./sec down to temperatures of from 500 to 800xc2x0 C.
The step of coiling the cooled steel strip comprises coiling the secondary-cooled steel strip at temperatures of from 400 to 750xc2x0 C.
In the method for manufacturing steel sheet, when a sheet bar is formed by rough-rolling a continuously cast slab containing more than 0.8% and not more than 1% C by weight, the sheet bar is finish-rolled at finishing temperatures of not less than (Acm transformation point xe2x88x9220xc2x0 C.).
Secondly, it is an object of the present invention to provide a method for manufacturing steel sheet that induces less failures in forming to a product shape, is possible to conduct product layout on a coil at high yield, has superior workability of stretch flanging performance and breaking elongation, has high shock resistance, and gives excellent tensile strength as high as 340 MPa or more.
To attain the object, the present invention provides a method for manufacturing steel sheet comprising the steps of: forming a slab; forming a hot-rolled steel sheet; applying primary cooling and secondary cooling to the hot-rolled steel sheet; and coiling the cooled steel sheet.
The step of forming the slab comprises continuous casting to give treatment for reducing segregation to manufacture the slab consisting essentially of 0.05 to 0.14% C, 0.5% or less Si, 0.5 to 2.5% Mn, 0.05% or less P, 0.1% or less S, 0.005% or less O, and less than 0.0005% Ca, by weight.
The step of forming the hot-rolled steel sheet comprises hot-rolling the slab at finishing temperature of finish-rolling not less than Ar3 transformation point.
The primary cooling step comprises cooling the hot-rolled steel sheet starting the primary cooling within 2 seconds after the hot-rolling to temperatures of from 600 to 750xc2x0 C. at cooling speeds of from 100 to 2,000xc2x0 C./sec.
The secondary cooling step comprises cooling the primary-cooled steel sheet starting the secondary cooling to the above-described temperature range at cooling speeds of less than 50xc2x0 C./sec. The secondary-cooled steel sheet is coiled at temperatures of from 450 to 650xc2x0 C.
Thirdly, it is an object of the present invention to provide a method for manufacturing steel sheet that provides wanted strength characteristics stably.
To attain the object, the present invention provides a method for manufacturing steel sheet comprising hot-rolling step and cooling step.
The step of hot-rolling comprises hot-rolling a steel consisting essentially of 0.03 to 0.12% C., 1% or less Si, 5 to 2% Mn, 0.02% or less P, 0.01% or less S, at least one element selected from the group consisting of 0.005 to 0.1% Nb, 0.005 to 0.1% V, and 0.005 to 0.1% Ti, by weight, at temperatures of 1,070xc2x0 C. or below to accumulated reductions in thickness of 30% or more.
The step of hot-rolling may be carried out on a steel consisting essentially of 0.03 to 0.12% C, 1% or less Si, 0.5 to 2% Mn, 0.02% or less P, 0.01% or less S, and 0.05 to 0.5% Mo, by weight, at temperatures of 1,070xc2x0 C. or below to accumulated reductions in thickness of 30% or more.
The step of cooling comprises cooling steel sheet within 6 seconds after the completion of the rolling to temperatures higher than 500xc2x0 C. and not higher than 700xc2x0 C. at average cooling speeds of not less than 80xc2x0 C./sec.